add testing suite for length difference function

Makefile

@@ -2,6 +2,14 @@ CFLAGS=-O3 -Iinc/
 
 build: main
 
+test: tester
+
+tester: out/tester.o out/swapper.o
+	gcc $(CFLAGS) out/tester.o out/swapper.o -o tester
+
+out/tester.o: out/ src/tester.c
+	gcc $(CFLAGS) -c src/tester.c -o out/tester.o
+
 out/:
 	mkdir out/
 
@@ -15,6 +23,6 @@ out/swapper.o: out/ src/swapper.c
 	gcc $(CFLAGS) -c src/swapper.c -o out/swapper.o
 
 clean:
-	rm -r main out/
+	rm -r main tester out/
 
-.PHONY: build clean
+.PHONY: build test clean